1. Field of the Invention
This invention generally relates to a time base correcting apparatus and more particularly relates to a time base correcting apparatus capable of correction of time base errors or variations contained in a digital signal which is formed of successive data blocks.
2. Description of the Prior Art
It is known that, when a signal such as a video signal, an audio signal and so on is modulated or converted into a pulse code modulation (PCM) signal to be recorded and reproduced, a time base correcting apparatus or corrector (hereinafter, simply referred to as TBC) is used in the playback side. The essential function of the TBC is to remove time base errors or variations contained in a reproduced data. To this end, the reproduced data is written in a memory by a clock pulse in synchronism therewith and then read-out therefrom by a reference clock pulse. When recording and reproducing the audio PCM signal and so on, an error correctable encoding is carried out, PCM data of plural words and error detection (or correction) codes are used to constitute one block and a synchronizing signal is added to each block so that the data of each block is written in the memory of TBC at its predetermined address. Since the output data from the TBC is supplied to an error correction decoder, a digital-to-analog (D/A) converter and so on, thereby error correction, D/A conversion and so on being conducted. For the purpose of performing these processings correctly, time series of the output data from the TBC must be coincident with that of the reproduced data.
By the way, the conventional TBC is provided, forming the write-in address on the basis of the synchronizing signal separated from the reproduced data. Since the synchronizing signal contained in the reproduced data is affected by a dropout, a noise and so on, the control of the write-in address based on such synchronizing signal is not made stable and hence the data of one block is written at the wrong block address with a result that the time series of data read out from the memory becomes different from that of the reproduced data. Particularly when an interleave technique is used to cope with burst errors due to dropout, the time series of the reproduced data must strictly be coincident with that upon recording. When the synchronizing signal is separated from the reproduced data, in the conventional TBC, it is proposed to eliminate the influences of dropout and noise and then to derive only the correct synchronizing signal, too. But, this is not satisfactory.
Therefore, we, the same assignee of this application, have previously proposed a time base correcting apparatus capable of performing the correct control of the write-in address by adding the block address to every blocks and writing the data of each block in the address of the TBC corresponding to this block address, which apparatus is disclosed in the U.S. patent application Ser. No. 06/298,522, filed Sept. 1, 1981.
By the way, in a tape recorder to which the TBC is applied, discontinuity is caused in a phase of a servo control signal recorded on a magnetic tape due to splice-edition and so on. Thus, a phase-locked loop (PLL) servo, by which upon playback the running of the magnetic tape is phase-locked to the reference signal, is made unstable by this discontinuity giving rise to a problem that it takes a long settling time for the magnetic tape to be phase-locked to the reference signal. In order to solve such problem or defect, there is employed a servo circuit in which a polyphase, for example, four-phase signal is used or a reference signal so as to enable the transportation of the magnetic tape to be phase-locked to any one phase of the four-phase reference signal. When this polyphase servo circuit is employed, there exist plural phase relations between the reference signal and the reproduced data. Since the read-out address of the aforenoted TBC is formed from the reference signal, depending on the phase relation of the incoming data, a phenomenon as if the time base errors are increased may take place with the result that the correction ability of the TBC will be sometimes lowered substantially.